Modeling a Working Clock
As many have stated above, getting a 'working' clock in blender is possible, although it highly depends on your definition of working. 

You can:


manually animate all moving parts using keyframes.
animate all moving parts using drivers.
be fancy, animate one moving part using a driver and link the movement of all the other parts to it's movement, using constraints.


You can't simulate the clocks physics in blender in order to let it drive itself (or at least you probably can't, in any case it would be extremely difficult). 

Reasons:


(as far as I know) the physics in Blender are not designed for accuracy and continuum mechanics
models in Blender are not parametric but meshes. That means things like the forces transmitted between your gears will not be calculable by means of simple polynomial or differential equations, but rather FEM (don't even bother with that stuff).


TL;DR: Do, what all the comments above suggested or be ready for a major headache.